Glass structure as a driver of polaronic conductivity in phosphate glasses containing MoO3 and WO3

Abstract

Glasses containing transition metal (TM) oxides exhibit polaronic conductivity which, according to a well-accepted view, depends on the overall amount of TMO and the fraction of reduced TM ions. In this study we show that in phosphate glasses containing WO₃ and MoO₃, polaronic conductivity is entirely governed by the structural features of the glass network and cannot be correlated with these parameters. Tungsten glasses are found to exhibit six orders of magnitude higher conductivity than their molybdenum counterparts despite a significantly lower fraction of W⁵⁺ ions and the compositional change of DC conductivity is non-monotonic in both glass systems. We relate the former effect to the tendency of WO₆ units to aggregate by forming W–O–W–O–W bonds which facilitate polaronic transport, in contrast to MoO₆ and MoO₄ units which are prone to incorporate into a phosphate network uniformly thus failing to provide such fast conduction pathways. More interestingly, we show that the latter effect originates from the manner in which tungstate or molybdate units are mutually linked within the phosphate network: if they form chains with a sufficient number of flexible terminal oxygen bonds (lower TMO content) polaronic transport is facilitated whereas if they connect three-dimensionally, causing a reduction in the number of terminal oxygens (higher TMO content), conductivity is hindered. A detailed analysis of frequency-dependent conductivity and permittivity using the MIGRATION concept helps us understand the structural features that influence conductivity in these glasses and points out the importance of network connectivity in the propagation of polarons on long-range and localized scales.